In antenna measurements, particularly in near-field measurements, the acquisition of two orthogonal field components is essential. Normally, this is done by rotating a field-probe by 90° around its central axis, typically an open ended waveguide or horn antenna. The first polarization of an antenna under test is measured by the probe in the first orientation of the antenna under test, and the orthogonal polarization can be acquired in the second orientation.
However, turning the probe requires the feed line to be moved also. This induces some undesired amplitude and/or phase errors. For higher frequencies the losses normally become undesirably high.
U.S. Pat. No. 5,781,087 discloses a rectangular waveguide rotary joint that allows limited mechanical rotation of two rectangular waveguides around a common longitudinal axis. The joint comprises a first rectangular waveguide having a first waveguide flange and a second rectangular waveguide having a second waveguide flange, wherein the second waveguide flange is disposed adjacent to the first waveguide flange with an air gap disposed there between. An RF choke is formed in the waveguide flanges for reducing RF leakage caused by the air gap, and a low friction spacer system for separating the first and second waveguides to maintain relative alignment of the waveguides during rotation and maintain a substantially constant separation between the waveguides.
For higher frequencies, especially frequencies above e.g. 50 GHz, there can be rotary joints in which the fundamental mode in a rectangular waveguide is transformed to a rotationally symmetric mode in a circular section. This transformation must be employed on both sides of a circular waveguide section. Thus, the losses are relatively high and the mechanical dimensions are very bulky.